Engine retarders of the compression release-type, also known as engine brakes, are well-known in the art. Engine retarders are designed to convert at least temporarily, an internal combustion engine of compression-ignition type into an air compressor. In doing so, the engine develops retarding horsepower to help slow the vehicle down. This can provide the operator increased control over the vehicle and substantially reduce wear on the service brakes of the vehicle. A properly designed and adjusted compression release engine retarder can develop retarding horsepower that is a substantial portion of the operating horsepower developed by the engine in positive power.
Functionally, compression release retarders supplement the braking capacity of the primary vehicle wheel braking system. In so doing, they may extend substantially the life of the primary (or wheel) braking system of the vehicle. The basic design for a compression release engine retarding system without exhaust gas recirculation is disclosed in Cummins, U.S. Pat. No. 3,220,392, issued November 1965.
The compression release engine retarder disclosed in the Cummins '392 patent employs a hydraulic system or linkage. The hydraulic linkage of the compression release engine retarder may be linked to the valve train of the engine. When the engine is under positive power, the hydraulic linkage may be disabled from providing the valve actuation that provides the compression release event. When compression release retarding is desired, the hydraulic linkage is enabled such that the compression release valve actuation is provided by the hydraulic linkage responsive to an input from the valve train.
Compression release occurs by opening the exhaust valve at a point near the end of a piston's compression stroke. In doing so, the work that is done in compressing the intake air cannot be recovered during the subsequent expansion (or power) stroke of the engine. Instead, it is dissipated through the exhaust and radiator systems of the engine. By dissipating energy developed from the work done in compressing the cylinder gases, the compression release retarder dissipates the kinetic energy of the vehicle, which may be used to slow the vehicle down.
Among the hydraulic linkages that have been employed to control valve actuation (both in braking and positive power), are so-called "lost-motion" systems. Lost-motion, per se, is not new. It has been known that lost-motion systems are useful for variable valve control for internal combustion engines. In general, lost-motion systems work by modifying the hydraulic or mechanical circuit connecting the actuator (typically the cam shaft) and the valve stem, to change the length of that circuit and lose a portion or all of the cam actuated motion that would otherwise be delivered to the valve stem to institute a valve opening event. In this way lost-motion systems may be used to vary valve event timing duration, and the valve lift.
Compression release engine retarders may employ a lost motion system in which a master piston engages the valve train (e.g. a push tube, cam, or rocker arm) of the engine. When the retarder is engaged, the valve train actuates the master piston, which is hydraulically connected to a slave piston. The motion of the master piston controls the motion of the slave piston, which in turn may open the exhaust valve of the internal combustion engine at the appropriate point to provide compression release valve events. In order to properly carry out the compression release events, it is necessary to reset (close) the valve in between the various valve events. If the valve is not reset, relatively small displacement events, such as compression release, may not be carried out.
One way of resetting the exhaust valve when using a unitary cam lobe for compression release valve events is to limit the motion of the slave piston which is responsible for pushing the valve into the cylinder during compression release events. A device that may be used to limit slave piston motion is disclosed in Cavanagh, U.S. Pat. No. 4,399,787 (Aug. 23, 1983) for an Engine Retarder Hydraulic Reset Mechanism, which is incorporated herein by reference. Another device that may be used to limit slave piston motion is disclosed in Hu, U.S. Pat. No. 5,201,290 (Apr. 13, 1993) for a Compression Relief Engine Retarder Clip Valve, which is also incorporated herein by reference. In theory, both of these valves (reset and clip) may comprise means for blocking a passage in a slave piston during the downward movement of the slave piston. After the slave piston reaches a threshold downward displacement, the reset valve or clip valve may unblock the passage through the slave piston and allow the oil displacing the slave piston to drain there through, causing the slave piston to return to its upper position under the influence of a return spring.
As the market for lost motion-type compression release retarders has developed, engine manufacturers have sought ways to improve compression release retarder performance and efficiency. Environmental restrictions, in particular, have forced engine manufacturers to explore a variety of new ways to improve the efficiency of their engines. These changes have forced a number of engine modifications. Engines have become smaller and more fuel efficient. Yet, the demands on retarder performance have often increased, requiring the compression release engine retarder to generate greater amounts of retarding horsepower under more limiting conditions.
The focus of engine retarder development has been toward a number of goals: securing higher retarding horsepower from the compression release retarder; working with, in some cases, lower masses of air deliverable to the cylinders through the intake system; and the inter-relation of various collateral or ancillary equipment, such as: silencers; turbochargers; and exhaust brakes. In addition, the market for compression release engine retarders has moved from the after-market, to original equipment manufacturers. Engine manufacturers have shown an increased willingness to make design modifications to their engines that would increase the performance and reliability and broaden the operating parameters of the compression release engine retarder.
One way of increasing the braking power of compression release engine retarders is to carry out exhaust gas recirculation (EGR) in combination with the compression release braking. Exhaust gas recirculation denotes the process of briefly opening the exhaust valve at the beginning of the compression stroke of the piston. Opening of the exhaust valve at this time permits higher pressure exhaust gas from the exhaust manifold to recirculate back into the cylinder. The recirculated exhaust gas increases the total gas mass in the cylinder at time of the subsequent compression release event, thereby increasing the braking effect realized by the compression release event.
It has been found that the exhaust gas recirculation event may be partially or totally lost as a result of unintentional resetting of the slave piston using a system that employs a Cavanagh type reset valve. Accordingly, there is a need for system, and method of operation thereof, that deactivates the reset for EGR events. There also remains a significant need for a system and method for controlling the actuation of the exhaust valve in order to increase the effectiveness of resetting to optimize the compression release retarding event.
A proposed system for carrying out compression release retarding and exhaust gas recirculation is disclosed in U.S. Pat. No. 5,146,890 to Gobert et al. ("Gobert"). The system disclosed in Gobert utilizes a two position device incorporated into the engine valve train between the cam and the valve stem. The device provides two distinct lash positions; one for positive power, and one for engine braking. During positive power the engine retarder is off, the device is retracted, and the relatively small compression release and exhaust gas recirculation events are "lost" due to the lash between the retracted device and the remainder of the valve train. When the engine retarder is turned on, the device extends to take up the lash in the valve train. Taking up the lash results in transmission of the compression release and exhaust gas recirculation lobes on the cam through the entire valve train to the valve stem.
FIG. 1 illustrates exhaust valve motion that occurs using the Gobert system during positive power (dashed line A) and during engine braking (broken line B). By taking up the lash during engine braking, the Gobert system produces a larger main exhaust valve event 50 than would otherwise be realized. The larger main exhaust event increases valve lift, duration, and increases the overlap between the main exhaust event 50 and the main intake event 60. The increase in exhaust-intake overlap is illustrated by shaded area 65 in FIG. 1. Increased overlap may be undesirable because it allows air that is normally trapped in the cylinder for a subsequent compression release event to escape from the cylinder past the open exhaust valve. A larger main exhaust event may also be undesirable because it could cause the exhaust valve to impact with the piston.
Gobert suggests that the increased overlap, that occurs inherently as a result of using the Gobert system, may be controlled by intentionally decreasing the size of the main exhaust and the main intake valve events during engine braking. See, column 2, lines 58-64 of Gobert. Hypothetically, the cam profiles could be reduced to produce main exhaust and main intake valve events of the desired magnitude during engine braking. With reference to FIG. 2, this change would inherently produce main exhaust 50 during positive power of lesser magnitude than the main exhaust event 50 during engine braking. Thus, if the main exhaust event is of the desired magnitude 54 during engine braking, then it is too small during positive power. If the main exhaust event is of the desired magnitude 52 during positive power, then it is too large during engine braking. A system is needed that can provide combination compression release and exhaust gas recirculation events and that can provide main exhaust and main intake events of a constant desired magnitude during positive power and engine braking.